Types of “Z-Girt” Sub-Framing Material
Z-girts and fasteners play crucial roles within the building envelope: Z-girts serve as attachment points for cladding, assist in establishing thermal breaks, and ensure a level surface for cladding installation, while fasteners maintain structural integrity by anchoring the Z-girt in place. This sub-framing is expected to withstand the rigors of long-term load-bearing and the fluctuations in building envelope temperatures. A cohesive selection of Z-girt material and fastener type is therefore essential for guaranteeing the long-term reliability of the structure.
Three materials were tested, chosen not only for their standing in contemporary construction practices, but also for the value of their comparative analysis.
- Fiber-Reinforced Polymer (FRP): A versatile thermoset plastic well-suited for use in both new construction and repair applications. As a composite integrating fibrous materials within a polymer matrix, FRP addresses many of the drawbacks of materials like wood, steel, concrete, and masonry (1) through its high strength-t0-weight ratio, corrosion-resistance, cost-effective3ness, and thermal insulative properties.
- Steel: Steel, with its established robustness, serves as a benchmark for structural integrity and durability that all sub-framing materials should aim to match or surpass to ensure safety and long-term performance. It is a common material with high load-bearing capacity at the cost of susceptibility to corrosion and high thermal conductivity, leading to potential thermal bridging. It offers screw pull-out resistance due to its tensile strength and inherent ductility.
- GreenGirt® Composite Metal Hybrid (CMHᵀᴹ): A high-performance building material that combines the corrosion-resistant and insulative properties of FRP with the structural resilience provided by a continuous metal component. Its unique composite ensures robust screw retention. This allows screws to be used directly, tapping into a continuous metallic structural support for effective and reliable load distribution.
This paper is intended to provide an accessible yet technically accurate overview of the physical processes that occur when these materials are subjected to elevated temperatures and sustained loads. Due to FRP’s performance in our testing, it will serve as the primary topic of discussion, demonstrating the necessity of industry-specific guidelines that accommodate anisotropic behavior, thermal constraints, and creep resistance.
The recommendations provided in the conclusion are straightforward and grounded in empirical data. Our goal is to furnish stakeholders with actionable insights that not only enhance the longevity of FRP constructions, but also identify and mitigate potential liabilities, ensuring more safe, durable, and responsible building practices.
We will discuss why, in structural or semi-structural applications, a self-drilling screw anchored into an FRP substrate is a short-term connection, whereas a metal-to-metal connection using steel or CMH is the best long-term fastening solution.
FRP Fundamentals
The history of FRP in structural applications dates to the early 20th century. Initially, FRP gained attention for its use in aerospace and marine engineering, and by the 1960s and 1970s found its way into civil construction. (2) When used correctly, FRP is regarded as a reliable structural material that offers a versatile alternative to traditional construction materials like steel and concrete.
FRP¹ can be created in various ways, including a process called pultrusion. Invented in 1959 (3) by W. Brandt Goldsworthy, the pultrusion machine converts continuous lengths of reinforced fibers and liquid resin into a fiber-reinforced plastic using a heated forming die. The fibers, which can be glass, carbon, or aramid, provide the material with its mechanical strength. The polymer matrix, here illustrated for the sake of clarity as two distinct layers, is infused throughout the glass fibers, acting as the “glue” that binds them together. This infusion ensures that stress is evenly distributed along the composite, and it also imparts shape to the material.
¹ This paper will be discussing Glass Fiber Reinforced Polymer (GFRP), which for the sake of simplicity will be referred to as FRP.
Figure 1: Depiction of the layers within a generic FRP composite. Note that the resin will saturate the assembly in its entirety.
Two of the primary fibers used in pultrusion are the continuous filament mat and fiberglass roving. While the continuous filament mat is made up of randomly oriented molten glass fiber strands, the roving is a strand of fiberglass that has been repeatedly doubled back on itself lengthwise. Typically, rovings comprise the bulk of reinforcement, conferring increased lengthwise strength and stiffness.
Factors Affecting Durability: Service Temperatures and Connection Details
Service Temperatures
We all know that getting to a car left parked in the sun on a hot summer’s day can feel like climbing into an oven. Vehicles parked outdoors in the sun, with ambient temperatures between 72 – 99°F (22 – 37°C), have been found (4) to reach interior ambient temperatures of 196°F (91°C ) after just 1-2 hours in the sun. Moreover, surface temperatures surpassed even the interior air temperatures, with dark dashboards registering temperatures over 220°F (104°C). Cars have been found to have similar heat gain to that of a building envelope. This demonstrates how quickly extreme temperatures can be reached and what service temperature range should be considered adequate for a building service envelope. (5) Like a car’s interior, a building’s envelope mediates the temperature interaction between the exterior and interior, playing an important role in maintaining comfort, energy efficiency, and structural longevity. Service temperatures are integral in determining the appropriate use and application of various construction materials, ensuring optimal function and structural integrity over time.
The building envelope, which serves as the protective barrier of a structure against external elements, operates within a service temperature range of -40°F to 180°F.² (6) This system can undergo significant heat gain or buildup and includes diverse materials across the exterior wall assembly — from sheathing and waterproofing to exterior insulation, sub-framing, and cladding. A key observation is that building envelope service temperatures can surpass ambient temperatures by up to 90°F. This can be a point of concern, particularly if FRP profile manufacturers haven’t tailored their products for such elevated temperatures, as FRP is commonly designed with only ambient temperatures in consideration.
² ASHRAE and IECC provide temperature ranges that are generally accepted and designed for: 55 to 85 degrees Fahrenheit for interior building components (7) and -40 to 120 degrees Fahrenheit for exterior ambient temperatures (8)
It’s also worth noting that a considerable amount of sub-framing is situated within the building envelope. This placement means it experiences the extensive temperature fluctuations present in the -40°F to 180°F range, underscoring the importance of choosing materials robust enough to handle these variates in the long term.
Figure 2: Heat distribution through a building envelope cavity at various service temperatures.
Connection Details
A crucial element of sub-framing integrity is the way that it’s connected. When using generic FRP, best practice calls for using backer plates along with bolt or screw assemblies. Established guidelines, such as those from the ASCE (9), have been codified for over 50 years and are considered best practice. They emphasize the significance and methodology of these connection details, which have been validated over time. There are several technical advantages to these established practices:
- Laminate Compression: The use of these methods effectively compresses the drilled hole, ensuring that the laminate layers are securely bound together. This not only strengthens the connection point but also enhances the overall stability of the structure.
- Force Distribution: By incorporating backer plates and bolt/screw assemblies, the force is uniformly distributed over a larger tributary area, surpassing what would be achieved by just a hole. This broader distribution, often referred to as the “X” area, minimizes localized stress points, further reinforcing the connection.
- Stress Redirection: One of the notable feats of this connection method is its ability to redirect stresses. Instead of concentrating them in the interlaminar zones, which might be weaker, the stresses are shifted to the stronger longitudinal and transverse directions of the laminate. This redirection not only optimizes the load-bearing capacity and the connection but also prolongs its lifespan.
It’s worth noting that these connection methodologies aren’t just theoretical best practices — they have been proven and refined over 50 years. Furthermore, GreenGirt® CMHᵀᴹ inherently aligns with these best practices owing to the continuous backer plate integrated into its design. This not only enhances its strength but also automatically facilitates the compression, distribution, and redirection of forces.
Joining FRP with screws or bolts comes with special engineering considerations:
• Use a backer plate for hole-drilling to minimize backside delamination damage or “hole push-out.”
• Install bolts and/or screws with washers to backer plates to distribute fastener load.
• Utilize established ASCE best practices to minimize risk of screw pull-out and ensure structural integrity.
GreenGirt® CMHᵀᴹ design accounts for these established engineering guidelines for joining FRP:
• The continuous steel backer plate on the cladding side flange serves as a hole-drilling backup plate while the self-drilling fastener drills first through cladding, then FRP, and finally the continuous steel backer plate. Self-tapping threads bite first into the steel insert, guiding the screw through the FRP.
• The installed cladding attachment screw structurally connects to the continuous steel backer plate within a lattice-like network of steel inserts that distribute tributary loads.
• The backer plate on the substrate side flange for CMH serves as protection against pull-over/pull-through of fasteners when going through FRP.
Short-Term Connections: Sheet Metal Screw to FRP Substrate
Why isn’t a screw without a backer plate compatible with the material properties of FRP? We’ll explain the chain of events.
1. Sheet Metal Screws Anchored Directly into FRP: Damaging the Composite Structure and Matrix
Among the common fastening methods — nails, screws, bolts, and adhesive — self-drilling or self-tapping screws have become a standard in building construction, although their successful use largely depends on the materials they’re drilling and fastening into. Self-drilling fastener manufacturers design their products primarily for metal and wood applications, and provide guidelines based on their materials.
Utilizing self-drilling/tapping screws based on misapplied guidelines leads to significant drawbacks – namely, that the structure loses integrity the moment that the screw enters the FRP. The issue arises from the incompatibility between the design intent of standard screw threads and the anisotropic mechanical behavior of FRP materials. The thread design of the fastener, geared for biting into metal, disrupts the fibers that give FRP its strength.³ Finite Element Analysis (FEA) reveals that these localized stresses can result in the initiation and propagation of cracks, especially if the composite has any pre-existing imperfections. (10) This drastically reduces the composite’s strength against fatigue and its long-term performance. (11)
Figure 3: Artist’s rendering of a micrograph (12) depicting FRP composite undergoing screw pull-out. In practice, drilling causes more push-out delamination than peel-up. (13)
³ In a typical metal-to-metal fastening scenario, the helical geometry of screw threads is designed to form a close interlock with isotropic materials like steel or aluminum. The threads bite into the material, creating a stable mechanical joint that benefits from the homogeneous nature of the metal. The shear stress is evenly distributed, and the material’s mechanical properties are consistent in all directions. Yet when driving a screw into FRP, as an anisotropic composite material, its threads generate localized shear forces that the anisotropic nature of FRP is ill-equipped to handle uniformly. The threads directly disrupt and damage the fibrous rovings. This disruption is commonly referred to as “fiber kinking” or “interfacial debonding.” This local damage consequently weakens the surrounding matrix.
These issues are summarized by Dr. Mahmood Haq, Civil & Structural engineering professor at Michigan State University: “Drilling a hole in composites sacrifices up to 60 percent of the load carrying capacity. The minute you drill a hole, the fibers become discontinuous, you create stress concentrations around the hole, and you cause delamination, all of which act as failure initiation points, thereby weakening the resulting joints.” (12)
2. Sheet Metal Screws Anchored Directly into FRP: Creating Force Concentrations
The introduction of a screw creates a significant force concentration. What begins as a low-to-medium stress in a broad tributary area of the composite gets concentrated into a much weaker interlaminar zone of the composite.
Let’s look at a real-life example. A typical fastener used to attach cladding is placed every 24 inches horizontally and vertically, covering a 4-square-foot area (or 576 square inches) for each fastener. Now, if wind applies a pressure of 20 pounds on every square foot of this area, each fastener will need to handle a load of 80 pounds. Usually, this is manageable, especially if a backer plate is used to spread out this load.
However, when fastening into a material like FRP, things change. Figure 3 shows that most of the stress from the load is focused on a small area around the screw. The area bearing this stress per thread can be found by with the following formula:
If the screw threads were to have an outer diameter of .25 and an inner diameter of .2, the equation would look like the following:
For the specific type of screw used in this example, this stress-bearing area per thread is about 0.0177 square inches. That means the 576 square inches of wind pressure is now focused on this small area around the screw, multiplying the force on it by a huge amount, between 8,000 to 30,000 times, depending on the number of screw threads examined. This shows how the material and fastening method can greatly affect how much stress is placed on a small area, which is crucial for ensuring the structure’s safety and durability.
This force concentration has severe implications. Even a fastener without any applied external load can exert an interlaminar force of up to 4,641 psi on the FRP. To put this in perspective, the overall interlaminar capacity of many FRPs ranges between 2,000 to 6,000 psi. This means that the fastener is already pushing the material to its limits, and with the addition of any external loads, the risk of failure increases substantially. Furthermore, the strength capacity of the screw thread will start to decline immediately due to the phenomenon known as screw creep. The preload applied to the fastener during installation can further exacerbate this situation, intensifying the interlaminar stresses and
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